1. Field of the Invention
The present invention relates to an antenna apparatus with earth for vehicles provided on a glass window pane of a vehicle or the like, and to the structure of ground or earth for an antenna.
2. Related Art
Vehicular window panes limit drivers' sight. To ensure visibility, various legal regulations are made on vehicular antennas provided on the window panes. Accordingly, the glass antennas must be designed within predetermined limitations regulations on size, width and length.
The above limitations on the shape of the glass antennas by the regulations have serious influence on performance of glass antennas which are already disadvantageous in comparison with rod antennas in point of sensitivity.
Known conventional glass antennas, such as a vehicular antenna device disclosed in Japanese Patent Publication No. 3-74845 and a loop-shaped glass antenna disclosed in Japanese Patent Application Laid-Open No. 3-1703, are not made with consciousness of the regulations, but are well-designed in antenna shape. Further, an antenna according to an article "Disc Shaped Mono-Pole Antenna with Wide Range and Directivity-less Reception Performance" (by Michiaki Ito and Hajime Seki) is known.
The vehicular antenna device in Japanese Patent Publication No. 3-74845 is made based on the relation between the antenna and a metal pillar. That is, the length of the pillar is set to .lambda./2 (.lambda.: wavelength of received wave) or shorter, while the length of the antenna is .lambda./4 or shorter. Since this prior art is made on the relation between the antenna and the pillar, and the antenna is basically a mono-pole type antenna, it includes no reference regarding a widthwise direction. Therefore, in consideration of present vehicle size, this antenna device is applicable to the FM band at most.
The glass antenna in Japanese Patent Application Laid-Open No. 3-1703 has a 200 to 1500 mm loop shape, a conductive wire is provided to connect the antenna at a feeding point in approximately parallel to a longitudinal central line of the vehicle. This prior art is made on the point of the total length of the loop antenna, however, the relation between a longitudinal length and a transversal length of the antenna is not considered. Accordingly, to clear the regulations and attain high reception performance, trials and errors with various lengths in the lengthwise and widthwise directions must be repeated within a range of 200 to 1500 mm.
The disk mono-pole type antenna as shown in the above article has a wide range performance with a round disk shape. However, this shape obstructs a driver's sight if the antenna is applied as a vehicular glass antenna. If the size of this antenna is reduced to ensure the driver's sight, a desired performance may not be attained.
Japanese Patent Laid-Open No. 50-24928 has proposed an antenna arranged behind a sideview mirror, although no proposal for the size of the antenna has been made.
Conventional antenna apparatuses for vehicles are also disclosed in Japanese Patent Laid-Open Nos. 61-82502 and 2-272804 and Japanese Utility Model Laid-Open No. 5-82113.
In Japanese Patent Laid-Open Nos. 61-82502 and 2-272804 and Japanese Utility Model Laid-Open No. 5-82113, screwing, adhesion, or a ground pattern is employed to ground the jacket line (shield line) of the feeder line of the antenna apparatus. For this reason, it is difficult to optionally attach an antenna earth structure to the vehicle.
Japanese Patent Laid-Open No. 5-152817 has proposed a ground wire provided along an antenna harness to prevent a decrease in reception sensitivity. In this prior art, a line soldered to the shield line of the feeder line is used as a ground wire, thereby facilitating attachment of the ground wire to the body. However, the earth structure of Japanese Patent Laid-Open No. 5-152817 has a ground wire set along the vehicle harness. That is, this structure assumes a harness provided near the antenna mounting portion and therefore cannot be applied to a vehicle without any harness near the antenna mounting portion. Even for a vehicle with a harness, it is difficult for the user to attach a ground wire to the harness.